A golf ball is mainly composed of a core and a cover covering the core, and an ionomer resin is mainly used as a base resin of the cover. Generally, the ionomer resin is an ionic copolymer of either three monomers, such as .alpha.-olefin, .alpha.,.beta.-ethylenically unsaturated carboxylic acid and metal .alpha.,.beta.-ethylenically unsaturated carboxylate, or four monomers such as .alpha.-olefin, .alpha.,.beta.-ethylenically unsaturated carboxylic acid, metal .alpha.,.beta.-ethylenically unsaturated carboxylate and .alpha.,.beta.-ethylenically unsaturated carboxylate, and various ionomer resins have been reported.
However, when the ionomer resin is used for a cover of current golf balls, a thickness of the cover is generally limited to 2.4 mm or less. This is because impact force which causes effects on shot feeling is compromised with impact resilience which causes effects on flight distance. If a design of a golf ball is made causing enlargement of flight distance, the impact force is naturally increased and the shot feel is bad, because the impact force when hitting is in proportion to the impact resilience. Therefore, the thickness of the cover was set at 2.4 mm or less as described above. Accordingly, there is a fixed limit to improve the shot feel to good by reducing the impact force and to improve the flight performances by enhancing the impact resilience.
Japanese Laid-Open Patent Publication No. 5 (1993)-123422 proposes that in order to enhance impact resilience, a thickness of the cover is made larger than that of the current solid golf ball while keeping impact force within a range which is acceptable. However, the above proposal does not simultaneously satisfy both the enhancement of impact resilience and the reduction of impact force.